The present invention relates to a steam turbine having brush seals between non-rotatable and rotatable components arranged and located to eliminate thermal bowing resulting from non-uniform distribution of heat about the rotatable component due to frictional contact between the brush seal and the rotatable component and particularly relates to apparatus and methods for eliminating axial thrust loads in the event of failure of the brush seal in such turbine.
In U.S. Pat. No. 6,168,377, of common assignee herewith, there is disclosed a steam turbine having a brush seal located between a non-rotatable component and a rotatable component of the rotor shaft. Particularly, axial flanges are provided on the dovetails of the buckets, the bucket dovetails being secured in complementary fashion to the dovetail of a rotor wheel. A brush seal comprised of an arcuate array of metal bristles projecting from the non-rotatable component toward the rotatable component, i.e., the flanges on the bucket dovetails, has bristle tips engaging with and bearing against the flange surfaces. As will be appreciated from a review of that patent, the contact between the bristles of the brush seal and the opposing sealing surface, i.e., the flanges, generates heat.
As disclosed in that patent, it was recognized that the contact between the brush seal and the sealing surface should be located radially outwardly of the rotor shaft in order to isolate the generated heat from the outer diameter of the rotor. Otherwise, the friction-generated heat may cause a non-uniform temperature distribution about the circumference of the shaft, resulting in non-uniform axial expansion of the rotor and, hence, a bow in the rotor. While various methods and apparatus are disclosed in that patent for eliminating that problem, one such solution located the friction-generating surface on the bucket dovetail flanges radially outboard of the outer shaft diameter. In that manner, the generated heat is isolated from the rotor, eliminating any tendency of the rotor to bow.
That patented design and other designs utilize conventional labyrinth-type packing seals on the inside of the diaphragm web as a backup to the brush seal. These labyrinth seals are located directly adjacent the outer diameter of the shaft. Brush seals are, however, susceptible to wear and failure. Should a brush seal spaced outwardly from the shaft fail, e.g., the brush seal of that patented design, the sealing diameter changes from the bucket dovetail platform to the rotor shaft. This, in turn, adversely changes the pressure distribution on the shaft and the thrust on the rotor in an axial direction. Accordingly, there is a need to provide a sealing system for a steam turbine in which not only is the problem of thermal bowing of the steam turbine rotor due to non-uniform heat distribution resulting from contact between brush seals and complementary sealing surfaces eliminated, but also the axial thrust loads on the rotor bearings are eliminated or minimized in the event of brush seal failure.
In accordance with a preferred embodiment of the present invention, there is provided a brush seal located radially outwardly of the outer diameter of the shaft of the rotatable component to eliminate thermal bowing of the rotor due to non-uniform heat distribution in combination with a labyrinth seal at substantially the same radial location to eliminate thrust loads in the event of failure of the brush seal. In this connection, the areas of axially opposed bucket dovetails defining axially opposite sides of the wheelspace between adjacent bucket wheels are substantially the same. The axially opposed adjacent bucket dovetails have axially extending platforms projecting from the bucket dovetails. The brush seal is located preferably on the downstream platforms of adjacent bucket stages with bristles projecting radially inwardly from the inner diameter of the diaphragm web. A pair of labyrinth teeth are similarly located on the upstream side of the web behind the brush seal. On the downstream side of the diaphragm web, a labyrinth seal is also provided with the axially projecting platforms of the axially adjacent buckets of the next adjacent downstream stage. The wheelspaces between the buckets are therefore generally symmetrical in a circumferential direction.
Should the brush seal fail, the labyrinth seals in these locations limit the performance degradation. The substantially equal axially opposed areas of the bucket dovetails also assist to prevent a change in rotor thrust from occurring. That is, since the areas of the upstream and downstream rotor surfaces exposed in the cavity inwardly of the diaphragm, i.e., the wheelspaces, are substantially equal, no net axial thrust from leakage flows past the labyrinth seal occurs. The platforms radially spaced from the rotor shaft thus provide sufficient heat dissipation to prevent induced heating from adversely affecting rotor vibrational performance, while the labyrinth seals in combination with the brush seals at substantially the same radial locations minimize or eliminate axial thrust loads in the event of brush seal failure.
In a preferred embodiment according to the present invention, a steam turbine comprising a rotatable component including a rotor shaft and a non-rotatable component about the rotatable component, a brush seal carried by the non-rotatable component for sealing engagement with the rotatable component, the rotatable component including a plurality of buckets having bucket dovetails for connection with complementary dovetails on a wheel of the rotatable component, means for inhibiting non-uniform circumferential heat transfer to the rotatable component thereby to eliminate or minimize bow of the rotatable component due to frictional contact between the brush seal and rotatable component, the inhibiting means including an axially extending platform on each the bucket dovetail in frictional engagement with the brush seal at a location spaced radially outwardly of an outer diameter of the rotor shaft, and a labyrinth seal carried by the non-rotatable component including at least one tooth extending between the non-rotatable component and the platform spaced generally radially outwardly from an outer diameter of the rotor shaft.
In a further preferred embodiment according to the present invention, there is a steam turbine comprising a rotatable shaft including a plurality of circumferentially spaced buckets at each of axially spaced locations therealong forming the buckets of adjacent stages of the turbine, a plurality of non-rotating partitions between the buckets of the adjacent stages including inner webs extending radially inwardly of a steam flowpath through the buckets and partitions, the buckets having bucket dovetails for connecting with complementary dovetails on wheels of the shaft, a brush seal carried by the inner webs, means for inhibiting non-uniform circumferential heat transfer to the rotatable shaft thereby to eliminate or minimize bow of the rotatable shaft due to frictional contact between the brush seal and rotatable shaft, the inhibiting means including a platform extending axially from each the bucket dovetail with portions of the platform lying radially inwardly of the inner webs and in radial alignment with portions of the inner webs, the brush seal extending from the inner web portions in frictional engagement with the platform portions in radial alignment therewith and at a location spaced radially outwardly of an outer diameter of the rotatable shaft, and a labyrinth seal carried by the inner web including at least one tooth extending between the inner web portions and the platform portions and spaced generally radially outwardly from an outer diameter of the rotor shaft.